The frontrunner after the preliminary round of Afghanistan's presidential election, Abdullah Abdullah, has strengthened his position in advance of next month's runoff, with endorsements from two failed but influential candidates.

Zalmai Rassoul and Gul Agha Sherzai, who have lined up behind Abdullah, have strong support in southern Kandahar province, where Abdullah has struggled to win votes partly because of his background. Both said they were backing Abdullah for the sake of national and ethnic unity, although they are also likely to have secured verbal promises of powerful government posts for their teams.

The son of Pashtun and Tajik parents, Abdullah spent years during the Soviet occupation and civil war at the side of the mujahideen commander Ahmad Shah Massoud, whose base was in the northern Panjshir valley. Abdullah's strongest support base is still in the north and centre of the country.

Pashtuns, who dominate the south and east of Afghanistan, have for centuries provided most of the country's rulers, including incumbent Hamid Karzai, and many are wary of where Abdullah's loyalties lie. Nationwide he claimed nearly 45% of the vote, but came fourth in Kandahar.

Rassoul, who announced his backing for Abdullah on Sunday, won Kandahar province, which is generally regarded as the Taliban's birthplace. Related to Afghanistan's deposed royal family, he was believed to have had Karzai's tacit backing in the first round.

Some key members of Rassoul's team were conspicuous by their absence when the alliance was announced, among them Karzai's two brothers Mahmoud and Qayoum, raising the possibility that Abdullah's rival in the runoff, Ashraf Ghani, may still woo them to his team for June's showdown.

Despite high turnout from determined voters, polling day also brought bloodshed and fraud, and there have been calls for the two candidates to broker a deal to avoid more violence. Both have so far insisted they are determined to see the next leader chosen at the ballot box.

Rassoul campaigned with a message of continuity and said he was backing Abdullah for the same reasons, along with Habiba Sarabi, the only female vice-presidential candidate in the race. "The main reason we have joined Dr Abdullah's team is to protect Afghan politics from ethnic divisions. We need national politics," said spokesman Javid Faisal.

He took to the stage in Kabul's plush Intercontinental hotel with Abdullah and fellow contender Gul Agha Sherzai, once the governor of Kandahar, who came in second in the province. Pictures of the trio linking hands in a purposeful show of ethnic unity raced round Facebook and Twitter within minutes of the news conference starting.

"There has been a lot of talk about the north and south dividing, a step like this will put an end to this gossip," said Sherzai's spokesman, Ahmad Zia Abdulzai.

Ghani has also won endorsements from important political players, but no one as high-profile as Rassoul. He is also lagging over 10 percentage points behind Abdullah in the first round, so if southern leaders are gathering behind a man once seen as the northern candidate, he may have a harder fight in June.